Conventionally, CRTs (Cathode Ray Tubes), EL (Electroluminescence) elements, PDPs (Plasma Display Panels), etc. have been put into practical use as displays of the light emissive type in which display contents can be overwritten electrically.
However, since this type of displays emit display light and use the same directly for the display, there arises a problem that their power consumption is quite large. Further since a light-emitting surface of the displays of this type serves as a display surface having high reflectance, if the displays of this type are used under the circumstances where ambient light is brighter than the luminance, for example, in direct sunlight, there always occurs a phenomenon known as "wash-out" in which the display light can not be observed.
On the other hand, liquid crystal displays have been put into practical use as color displays which display characters and/or images not by emitting the display light, but by adjusting an amount of transmitted light from a particular light source. These liquid crystal displays include a transmission type and a reflection type.
Of the two types, particularly popular are the liquid crystal displays of the transmission type which employ a light source called "back light" at the back side, namely, behind the liquid crystal cell. Since the liquid crystal displays of the transmission type are advantageous in thinness and lightness, they have been used in diversified fields. On the other hand, the liquid crystal displays of the transmission type consume a large amount of power to keep the back light turned ON. Thus, regardless of the advantage that only a small amount of power is consumed to adjust transmittance of the liquid crystal, a relatively large amount of power is consumed as a whole.
However, the liquid crystal displays of the transmission type (that is, color liquid crystal displays of the transmission type) wash out less frequently compared with the displays of the light emissive type. This is because, in the color liquid crystal displays of the transmission type, the reflectance on the display surface of a color filter layer is reduced by the reflectance reducing technique using a black matrix.
Nevertheless, it becomes too difficult to observe the display light on the color liquid crystal displays of the transmission type when they are used under the circumstances where the ambient light is very strong and the display light is relatively weak. This problem can be eliminated by using brighter back light, but this solution raises another problem that the power consumption is further increased.
Unlike the displays of the light emissive type and liquid crystal displays of the transmission type, the liquid crystal displays of the reflection type show the display using the ambient light, thereby obtaining display light proportional to an amount of the ambient light. Thus, the liquid crystal displays of the reflection type are advantageous in a principle that they do not wash out, and when used in a very bright place in direct sunlight, for example, the display can be observed all the more sharply. Further, the liquid crystal displays of the reflection type do not use the back light for the display, and therefore, have another advantage that the power for keeping the back light turned ON can be saved. For the above reasons, the liquid crystal displays of the reflection type are particularly suitable as the devices for the outdoor use, such as portable information terminals, digital cameras, and portable video cameras.
However, since these conventional liquid crystal displays of the reflection type use the ambient light for the display, the display luminance largely depends on the surrounding environment, and when used under the circumstances where the ambient light is weak, there arises a problem that the display content can not be observed. Particularly, in case that a color filter is used for realizing the color display, the color filter absorbs the light and the display becomes darker. Thus, when used under these circumstances, the above problem becomes more apparent.
To eliminate the above problem, a lighting device called "front light" has been developed as an auxiliary light, so that the liquid crystal displays of the reflection type can be used under the circumstances where the ambient light is weak. Since the liquid crystal displays of the reflection type have a reflection layer behind the liquid crystal layer, they can not use the back light as do the liquid crystal displays of the transmission type. For this reason, the lighting device (front light) lights the liquid crystal displays of the reflection type from the front side, that is, from the display surface side.
On the other hand, liquid crystal displays, employing a transflective film which transmits a part of incident light and reflects the rest, have been put into practical use as the liquid crystal displays which can be used under the circumstances where the ambient light is weak while maintaining the advantages of the liquid crystal displays of the reflection type. The liquid crystal displays using both the transmitted light and reflected light are generally referred to as the liquid crystal displays of the transflective type.
For example, Japanese Laid-open Patent Application No. 218483/1984 (Tokukaisho No. 59-21843) (Japanese Patent Application No. 92885/1983 (Tokugansho No. 58-92885)) discloses a liquid crystal display of the transflective type which modulates the brightness by the TN (Twisted Nematic) mode, STN (Super-Twisted Nematic) mode, etc., which are known as the liquid crystal display modes for modulating the luminance of the transmitted light. Also, Japanese Laid-open Patent Application No. 318929/1995 (Tokukaihei No. 7-318929) discloses a liquid crystal display of the transflective type, in which a transflective film is provided in close proximity to the liquid crystal layer. Further, Japanese Laid-open Patent Application No. 160878/1994 (Tokukaihei No. 6-160878) (U.S. Pat. Nos. 5,598,285 and 5,737,051) discloses a liquid crystal display of the transmission type adopting the in-plane switching method as a technique for realizing a wider range of viewing angles. However, since the liquid crystal display of the transflective type disclosed in Japanese Patent Application No. 218483/1984 (Tokukaisho No. 59-218483) has the transflective film behind the liquid crystal cell seen from the viewer's side, there occur the following problems (1) and (2).
(1) It is very difficult to set the brightness which affects a visibility of the display device. More specifically, when the brightness of the liquid crystal display of the transflective type is set adequately for the reflection display, the brightness is set high, so that it can be used under the circumstances where the ambient light is insufficient. However, if the brightness is set high by using a polarization plate having high transmittance in the TN method, for example, a contrast ratio, which is defined as a quotient obtained by dividing the brightness in the light display by the brightness in the dark display, becomes too low for the transmission display, thereby deteriorating the visibility. Conversely, when the brightness of the liquid crystal display of the transflective type is set adequately for the transmission display, it is preferable to set the brightness in such a manner as to raise the contrast ratio. However, in this case, the brightness becomes too low for the reflection display, thereby deteriorating the visibility as well.
(2) In the reflection display, since the display is observed by reflecting the light having passed through the liquid crystal layer sandwiched by the two substrates by the reflection film provided behind the liquid crystal cell, there occurs parallax (double image) and the resolution deteriorates, thereby making high-resolution display very difficult.
Also, in the liquid crystal display of the transflective type disclosed in Japanese Laid-open Patent Application No. 318929/1995 (Tokukaihei No. 7-318929), since the transflective film is used as the reflection film, there arises another problem that there is no optical design such that can be suitable for both the reflection display section and transmission display section.
Further, although the in-plane switching method disclosed in Japanese Laid-open Patent Application No. 160878/1994 (Tokukaihei No. 6-160878) is employed in the liquid crystal displays of the transmission type, the director configuration of the liquid crystal on the comb-shaped electrode does not contribute to the display. This is not because, in most cases, the electrode lines are made of metal that does not transmit light, but because the director configuration of the liquid crystal is not changed sufficiently for the transmission display.